In an electric storage device mounted on an electric vehicle (EV), a plug-in hybrid vehicle (PHEV), or a hybrid vehicle (HEV), in general, a plurality of electric storage cells are connected in series to configure an electric storage unit. When there is an individual difference of an electric storage cell such as fluctuation in a capacity or self-discharge fluctuation among the electric storage cells, fluctuation occurs in states of charge (SOC) of the electric storage cells included in the electric storage device. When this fluctuation occurs, charge control is performed with reference to the electric storage cell having the highest SOC among the plurality of electric storage cells and discharge control is performed with reference to the electric storage cell having the lowest SOC. Therefore, energy usable by the electric storage unit decreases. When it is anticipated that a range of use of an SOC becomes wide as in the PHEV or the EV, in a state in which the SOC is high or low, deterioration of the electric storage cell tends to progress. Therefore, it is necessary to take measures for, when the SOC is too high, reducing the SOC and, when the SOC is too low, preventing a further decrease in the SOC. Accordingly, in order to eliminate fluctuation in SOCs among a plurality of electric storage cells that could occur when the electric storage cells are connected in series, there is proposed a method in which a balancing circuit configured by a bypass resistor and a bypass switch connected to the electric storage cells in parallel and electric storage cell controller for monitoring states of the electric storage cells are mounted and the electric storage cell controller controls the bypass switch of the balancing circuit on the basis of a fluctuation amount of a voltage. This method is a balancing method for forcibly discharging an electric storage cell having a high voltage and performing equalization of the voltage.
JP-A-2004-31012 describes that, only when an average value of charging capacities of all cells of a assembly battery is within a range in which a gradient |ΔVo/ΔSOC| in a charging capacity SOC-open circuit voltage Vo characteristic of the cell is equal to or larger than a predetermined value, capacity adjustment conditions for the cells of the assembly battery are determined and capacity adjustment is performed according to the conditions and that, when the capacity adjustment is suspended, remaining information of the capacity adjustment is stored in storing means and, when the capacity adjustment can be performed, the capacity remaining information is read out from the storing means and the capacity adjustment is resumed.